Molecular sieves of the MFS framework type, and in particular ZSM-57, are useful catalyst components for a variety of conversion processes, such as hydrocarbon cracking, dehydrogenation, oligomerization, isomerization, disproportionation, and alkylation as well as the formation of hydrocarbons from oxygen-containing compounds such as alcohols and ethers.
The composition, properties and preparation of ZSM-57 are disclosed in European Patent No. 174,121, and U.S. Pat. Nos. 4,873,067 and 4,973,781, the entire disclosures of these documents being incorporated by reference herein. ZSM-57 is a zeolite with a typical molar ratio of YO2:X2O3 of at least 4, wherein Y represents silicon and/or germanium and X represents aluminum, and/or boron, and/or chromium, and/or iron, and/or gallium. Preferably, there are from greater than 8 to about 200 moles of YO2 per mole of X2O3. Preferably, YO2 is silica and X2O3 is alumina.
ZSM-57 may be prepared as described in European Patent No. 174,121 from a synthesis mixture containing sources of alkali metal ions, an oxide of silicon, an oxide of aluminum, water and an organic directing agent which is a salt of N,N,N,N′,N′,N′-hexaethylpentane diammonium (HEPD, also known as hexaethyl-Diquat-5), and maintaining said mixture under crystallization conditions until the required zeolite is formed. The synthesis mixture has a composition within the following ranges: SiO2:Al2O3 of 20 to 200:1, preferably 40 to 100:1; H2O:SiO2 of 10 to 200:1, preferably of 20 to 50:1; OH−:SiO2 of 0 to 3:1, preferably 0.1 to 0.5:1; Z:SiO2 of 0 to 3:1, preferably of 0.1 to 2:1, where Z is an alkali metal cation; R:SiO2 of 0.01 to 2:1, preferably of 0.1:1, where R is HEPD, preferably its dibromide salt. Crystallization of zeolite ZSM-57 may be carried out under either static or stirred conditions. A useful range of temperatures for crystallization is from 80° C. to 350° C. for a time of 12 hours to 200 days. Thereafter, the crystals are separated from the liquid and recovered. The synthesis of the zeolite crystals is said to be facilitated by the presence of at least 0.01 wt percent, preferably 0.10 wt %, and still more preferably 1 wt %, seed crystals (based on total weight) of crystalline product.
U.S. Patent Application No. 20050013774 A1 discloses a process for the manufacture of a crystalline molecular sieve of the MFS framework type, which comprises hydrothermal treatment of a synthesis mixture containing sources of alkali metal ions, of aluminum, and of silicon, water, an N,N,N,N′,N′,N′-hexaethylpentane diammonium salt (HEPD), hereinafter R1, and R2, an amine of formula NR1R2R3 or a quaternary ammonium compound of formula R1R2R3R4NX, wherein R1, R2, R3, or R4, which may be identical or different, each independently represent a hydrogen atom, a linear alkyl group having from 1 to 8 carbon atoms, a branched alkyl group having from 3 to 8 carbon atoms, or a cycloalkyl group having 5 or 6 carbon atoms, at least one of R1, R2, R3, and if present R4, being other than hydrogen, and X represents an anion.
U.S. Pat. No. 4,873,067 further illustrates the preparation of boron, chromium, iron and/or gallium-containing zeolite ZSM-57 by a method comprising preparing a mixture containing sources of alkali metal ions, an oxide of silicon, an oxide of aluminum, a source of boron, and/or chromium, and/or iron and/or gallium, water and HEPD, and maintaining said mixture under crystallization conditions until the required zeolite is formed.
Colloidal seeds have proved effective in controlling the particle size of the product, avoiding the need for an organic directing agent, accelerating synthesis, and improving the proportion of product that is of the intended framework type.
European Patent No. 1105348 provides a colloidal suspension of a LEV framework type crystalline molecular sieve and a method for preparing a colloidal suspension of LEV by synthesizing a LEV framework type crystalline molecular sieve by treatment of an appropriate synthesis mixture, separating the product from the synthesis mixture, washing the product, and recovering the resulting wash liquid.
The ZSM-57 material that is commercially available, however, is expensive and time consuming for manufacturing. There is therefore a need for an improved high throughput process for manufacturing the ZSM-57 material. We have now found that the throughput of the process for preparing the crystalline molecular sieves of the MFS framework type is improved by lowering the alkali and the water content of the synthesis mixture and/or by increasing the crystallization temperature.